Since a lithium battery, in particular lithium secondary battery, is characterized by a large energy density, long life span and the like, it is used as a power source of home electronic products such as video cameras, portable electronic devices such as notebook-type personal computers, cellular phone and the like, and recently, is also applied to large batteries to be on board of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
A lithium secondary battery is a secondary battery having a structure in which, at charging time, lithium melts out from the positive electrode as an ion and moves to the negative electrode to be stored, and at discharge time, conversely, the lithium ion returns from the negative electrode to the positive electrode; the high energy density thereof is known to originate from the electric potential of the positive electrode material.
As the positive electrode active material of a lithium secondary battery, lithium transition metal oxides such as LiCoO2, LiNiO2 and LiMnO2 having layered structure are known, in addition to lithium manganese oxide (LiMn2O4) having a spinel structure. For instance, LiCoO2 has a layered structure in which lithium atom layers and cobalt atom layers are stacked alternately via an oxygen atom layer, and since the charge-discharge capacity is large and diffusibility of lithium ion storage-unstorage is excellent, the majority of the currently commercialized lithium secondary batteries use LiCoO2 as a positive electrode active material, which has a high voltage of 4V. However, since Co is extremely expensive, development of lithium transition metal oxide having a layered structure (LiMxO2; M: transition metal), which may be a substitution material for LiCoO2, is desired.
In conventional art, as a lithium transition metal oxide having a layered structure (LiMXO2), an active substance represented by the formula LiNiXMn1−xO2 (where 0.7≦x≦0.95) is disclosed in Patent Reference 1, which is obtained by adding an alkaline solution into an aqueous mixed solution of manganese and nickel to coprecipitate manganese and nickel, adding lithium hydroxide and then firing.
In addition, a positive electrode active material represented by Li[LiX(APBQCR)1−x]O2 (where A, B and C are respectively different 3 species of transition metal elements, −0.1≦x≦0.3, 0.2≦P≦0.4, 0.2≦Q≦0.4 and 0.2≦R≦0.4) is disclosed in Patent Reference 2, comprising crystal particles of an oxide containing 3 species of transition metals, the crystal structure of the crystal particle being a layered structure, and the arrangement of the oxygen atoms constituting the oxide being cubic closest packing.
To provide a layered lithium nickel manganese complex oxide powder having high bulk density, a method for preparing a layered lithium nickel manganese complex oxide powder is disclosed in Patent Reference 3, whereby a slurry containing at least a lithium source compound, a nickel source compound and a manganese source compound, which have been ground and mixed, at a molar ratio [Ni/Mn] of nickel atom [Ni] and manganese atom [Mn] in the range of 0.7 to 9.0 is dried by spray-drying and fired to produce a layered lithium nickel manganese complex oxide powder, and then the complex oxide powder is ground.
A lithium transition metal complex oxide obtained by mixing vanadium (V) and/or boron (B) to increase the crystallite diameter is disclosed in Patent Reference 4, that is to say, a substance containing the lithium transition metal complex oxide represented by General Formula LiXMYOZ-δ (where M represents the transition metal element Co or Ni, and the relationships (X/Y)=0.98 to 1.02 and (δ/Z)≦0.03 are fulfilled) and at the same time containing with respect to the transition metal element (M) constituting the lithium transition metal complex oxide, vanadium (V) and/or boron (B) at ((V+B)/M)=0.001 to 0.05 (molar ratio), the primary particle diameter thereof being 1 μm or greater, the crystallite diameter being 450 Å or greater and the lattice distortion being 0.05% or less.
With the purpose of providing a positive electrode active material for non-aqueous secondary battery comprising a primary particle that maintains a high bulk density and the battery properties without concern about a crack occurring, a positive electrode active material for non-aqueous secondary battery is proposed in Patent Reference 5, which is a lithium complex oxide in powder form of monodisperse primary particle having as main components lithium and one species of element selected from the group comprising Co, Ni and Mn, in which the mean particle diameter (D50) is 3 to 12 μm, the specific surface area is 0.2 to 1.0 m2/g, the bulk density is 2.1 g/cm3 or greater and the inflexion point of the rate of volume decrease by the Cooper plot method does not appear until 3 ton/cm2.    [Patent Reference 1] Japanese Patent Application Laid-open No. H8-171910    [Patent Reference 2] Japanese Patent Application Laid-open No. 2003-17052    [Patent Reference 3] Japanese Patent Application Laid-open No. 2003-34536    [Patent Reference 4] Japanese Patent Application Laid-open No. 2004-253169    [Patent Reference 5] Japanese Patent Application Laid-open No. 2004-355824
Meanwhile, in contrast to batteries that are charged and discharged between the limit regions of the depth of charge-discharge as is the case for batteries of consumer products such as video cameras, notebook-type personal computers and cellular phones, since batteries that are to be on board of electric vehicles and hybrid vehicles are charged and discharged mainly in the middle region of the depth of charge-discharge (for instance, 50-80% SOC), to exhibit excellent battery properties when used in the middle region is desired of them, for instance, life span properties (cycle properties) and output properties.